Stethoscopes are used for listening to internal body sounds such as heart and lung sounds of humans as well as animals. To accomplish a proper diagnosis, the user must be able to hear low volume sounds and subtle sound patterns. In many instances, however, this may be difficult or impossible to achieve due to masking of the signal by ambient noise. Examples of situations in which proper use of traditional stethoscopes is difficult run the gambit from a noisy office or examining room to extreme environments such as in an ambulance or a medical evacuation helicopter.
There have been numerous attempts to increase the stethoscope signal level to overcome the problem with noise. Many of these proposed solutions are electronic stethoscopes which use a microphone element or another type of transducer to convert body vibrations into an electrical signal. The transducer is typically either placed in the bell of the chest piece that is placed on the body, or is fitted into a small section of tubing connected to the chest piece. The signal from the microphone is then amplified and either sent to a small loudspeaker which is connected through tubing to a traditional stethoscope earpiece, or used to drive a conventional set of headphones.
The object of the amplification is to increase the signal level presented to the ears of the stethoscope user. However, these devices are of limited usefulness in high noise environments because the transducer used to transduce the heart and respiration sounds also picks up and amplifies ambient noise. The available signal to noise ratio is determined by the configuration of the chest piece along with the ambient noise level, and no amount of amplification can increase the ratio. Any attempt to amplify desired sounds also amplifies noise. In addition, the earpieces and headsets used with these devices do not sufficiently attenuate ambient noise at the ear. Even if no noise was picked up by the chest piece transducer, noise that leaks through the ear pieces of headsets would mask the desired signals. The signal, in order to be perceived, needs to be amplified until it is louder than the leakage noise by at least 10 dB. Noise leakage through the headset limits the maximum noise level environment in which devices can be used. Only so much amplification can be used to increase the signal above the noise level before the risk of hearing damage becomes significant.